Technical Abstract:
Transgenic cattle carrying multiple genomic modifications have been produced by serial rounds of somatic cell chromatin transfer (cloning) of sequentially genetically targeted somatic cells. However, cloning efficiency tends to decline with the increase of rounds of cloning. It is possible that multiple rounds of cloning compromise the genome integrity or introduce epigenetic errors in the resulting cell lines, rendering cloning efficiency decline. To test these possibilities, we performed 9 high density array Comparative Genomic Hybridization (CGH) experiments to test the genome integrity in 3 independent bovine transgenic cell lineages generated from genetic modification and cloning. Our plan included the control hybridizations (self to self) of the 3 founder cell lines and 6 comparative hybridizations between these founders and their derived cell lines with either high or low cloning efficiencies. We detected similar amounts of differences between the control hybridizations (6, 9 and 41 differences) and the comparative analyses of both “high” and “low” cell lines (ranging from 4 to 45 with a mean of ~16). Since almost half of these differences shared the same type (loss or gain) and were located in nearby genomics regions, it is likely that they were not true differences but caused by systematic factors associated with local genomic features (e.g. GC contents). In summary, these findings reveal that large genomic structural variations are less likely to arise during genetic targeting and serial rounds of cloning, fortifying the notion that epigenetic errors introduced from serial cloning are responsible for the cloning efficiency decline.